1. Field of the Invention
This invention relates to inner shielding material disposed laterally around the electron beams of a color television picture tube such that it covers the electron beams, and a method of manufacturing the same.
2. Description of the Prior Art
The basic structure of a color television picture tube comprises an electron gun and a phosphor screen which transforms the electron beams into an image. Furthermore, the inside of the tube is covered with magnetic shielding material which prevents deflection of the electron beams due to the earth's magnetic field. The magnetic shielding material comprises a mask frame, shadow mask inner shielding, outer shielding and the like. The properties required of a magnetic shielding material include high magnetic permeability in the earth's magnetic field (a weak magnetic field of approximately 0.3 Oe) and, also, a low coercive force H.sub.C, which is necessary for improving the demagnetizing characteristics, specifically for reducing the number of turns of the demagnetizing coil and lower lowering its current. In particular, the inner shielding material disposed laterally around the electron beams inside a picture tube such that it covers the electron beams is particularly important as magnetic shielding material.
The material for the inner shielding is typically an extremely thin steel plate 0.10-0.25 mm thick, and this material (coil), after being press-worked by the electric equipment manufacturer, is subjected to magnetic annealing (700.degree.-850.degree. C.) if necessary and then a blackening treatment applied at a temperature of 550.degree.-600.degree. C., after which it is incorporated into the interior of the picture tube. The purpose of the blackening treatment is to improve the radiation of heat and prevent diffuse reflection of electrons.
However, in view of the expense of conducting two heat treatments (magnetic annealing and blackening treatment) electric equipment manufacturers are experimenting with omitting the magnetic annealing treatment to reduce costs, and one method of doing such is proposed in Japanese Published Unexamined Patent Application No. 60-255924. In this method, Al-killed steel sheet is tempered using skin pass rolling to coarsen the grains; the sheet is then subjected to strong cold rolling and a final blackening is used to recrystallize the sheet at the electric equipment manufacturer.
But blackening treatment carried out at the electric equipment manufacturer is expensive and in addition, blackening treatment done by the electric equipment manufacturer involves batch annealing after press working so the homogeneity of the blackened layer is a constant problem.